The critical temperature of a material is the highest temperature, at which the material can be liquefied from a gaseous state by raising the pressure. The critical pressure is the pressure required to affect liquefaction at the critical temperature.
A material can be brought to a supercritical state by first raising its pressure above the critical pressure, and then raising the temperature above the critical temperature. The state of a material under supercritical conditions differs both from the liquid and the gaseous state. Hereinafter, reference is made to a supercritical fluid. For example, the solvent characteristics of a supercritical fluid differ from those of the liquid in a normal state. Carbon dioxide in a supercritical state functions as a very efficient organic solvent. The solvent characteristics can be utilized in supercritical extraction. Supercritical fluid is directed through a material in a pressure vessel, whereby components are leached from the material into the fluid. When the pressure of the material is lowered to below the critical pressure, the solvent power of the fluid substantially changes, and dissolved material precipitates. When the liquid is evaporated, all the dissolved material is left in the residue. Supercritical extraction has gained popularity as the use of ordinary solvents has been decreased for environmental and occupational health reasons. Supercritical extraction is suitable for, e.g. separation of environmental toxins from polluted material. The extraction may also be used for separation of aroma components from aromatic herbs.
Supercritical extraction is useful in those cases where the material to be extracted is not reactive towards the supercritical fluid, and where the solubility of some of its components in the supercritical fluid is sufficiently high. As the solubility of organic compounds in the supercritical fluid generally is remarkably higher than that of inorganic compounds, the method has proved practical in the extraction of organic compounds from their mixtures with inorganic material.
By no means, however, is the solvent power of a supercritical fluid always sufficient to dissolve the desired compounds. When purifying polluted soil, the concentration of toxins may be too high even after extraction. The solvent power can be enhanced using different additives. When carbon dioxide is used, ethanol or methanol are common additives. Despite the use of additives, a satisfactory result is not always reached. As regulations for the protection of the atmosphere are globally getting tighter, new requirements are imposed on the quality of combustion engine fuels. Volatility and aromatics content are lowered, and lead-containing anti-knocking agents are phased out. As a consequence, the use of so-called alkylate fuel is rising. The term alkylate fuel refers to a hydrocarbon product normally produced by catalytic alkylation of isobutane with butene, propene or other olefins. A composition of branched hydrocarbons is thus produced, the octane number of which is above 90 without additives. Presently, sulfuric acid or hydrogen fluoride are used as catalysts, though research efforts are directed to the development of solid catalysts.
Above all the high cost of hydrogen fluoride and the high dryness requirements of the feed streams in processes utilizing it, processes catalysed by sulfuric acid are often preferred. The major drawback of these processes, however, lies in the regeneration of spent sulfuric acid. Worldwide use of sulfuric acid in the production of alkylated fuels comprises several million metric tons. The spent sulfuric acid consists of above 70%, normally at least 85-88% sulfuric acid into which a wide range of different organic compounds have dissolved. The composition of these compounds and the requirements as to the regeneration of the acid is strongly dependant on the impurities in the feed of the alkylation unit. The main impurity of the sulfuric acid bound for regeneration is so-called Red Oil, consisting mainly of long chain C.sub.12 -C.sub.16 hydrocarbons, and organic compounds originating from olefins polymerizing due to the acid or reacting with alkylate molecules, the reaction of a feed impurity diene with an olefin, water or a hetero compound, and from the reactions between other impurities and olefins or sulfuric acid. Stable esters arising from ethene are particularly harmful components from a process point of view. Usually the process is operated at a sulfuric acid strength of about 90%, whereby the acid contains about 8% Red Oil and about 2% water. The regeneration of sulfuric acid is normally carried out by incineration at about 1200.degree. C., whereupon the organic material is pyrolyzed. The generated sulfur trioxide is absorbed to produce new acid, which is recycled to the process at a concentration of about 98,5%, while a corresponding amount of diluted acid is withdrawn. Due to the high temperature incineration, the regeneration process is energetically disadvantageous. The adverse effects involved with the transport of spent sulfuric acid are obvious, and therefore the regeneration plant must often be situated in conjunction with the alkylation unit.